A laundry treating appliance is a common household device for treating laundry in accordance with a preprogrammed treating cycle of operation. A subset of laundry treating appliances uses a generally horizontally rotating drum to define a chamber in which the laundry is received for treatment according to the cycle of operation. The laundry may be distributed in the drum chamber in such a way as to introduce an imbalance, and increase the bending moment, acting on the rotating drive shaft.
Shaft bending moment will increase as the drum rotational speed increases for a given imbalance. Because the magnitude of drive shaft bending moment is generally limited by design to a selected maximum, the drive shaft or drum rotational speed must be limited. Generally, motor power or torque is used to estimate the shaft bending moment, with this information utilized to determine if a design limit bending moment has been reached. However, utilizing motor power or torque to calculate drive shaft bending moment can provide inaccurate results, and can necessitate calibration each time a cycle is operated, which increases overall cycle time and vibration level.
One solution for addressing the inaccuracies in the motor power and/or torque determinations is to establish a reduced design limit for acceptable bending moment values that provides a safety margin to ensure that normal operation will not result in a failure. As a result, when the bending moment is large, the drum must be rotated at lower spin speeds, or rotated to redistribute the laundry load. For example, if a maximum bending moment limit is 400 newton-meters, a factor of safety may be provided to limit the calculated moment to 350 newton-meters, which is sufficient to account for the magnitude of the errors in motor power or torque readings.